The study of quantum gravity has also spurred progress in classical gravity. One example of this is the discovery, using AdS/CFT, of the “fluid-gravity correspondence”, which shows that in certain regimes Einstein's equations reduce the Navier-Stokes equations of hydrodynamics [1]}. Many new solutions of Einstein's equations have also been found in the context of AdS/CFT, including black holes with scalar hair and stationary black holes with non-Killing horizons [2]}. There has been progress understanding possible astrophysical signatures of near-extremal Kerr black holes based on conformal symmetry [3]}, [4]}. There has also been progress on the cosmic censorship conjecture, in both the negative direction, showing that in the absence of the weak gravity conjecture Einstein Maxwell theory in \(AdS_4\)  can violate the conjecture [5]}, [6]}, and in the positive direction, showing that the Penrose inequality, which is a well-known stand-in for cosmic censorship, can be derived in AdS/CFT [7]}. An interesting connection between the Bondi, van der Burg, Metzner, and Sachs group of asymptotic symmetries in flat space, soft theorems, and the memory effect has also been discovered [8]}, and attempts to develop this into a theory of holography in flat space were summarized in the white paper [9]}. This involves an interesting reformulation of the S-matrix where the Lorentz group is realized as the conformal group of the sphere at infinity. The study of asymptotic symmetries has also motivated progress on the Hamiltonian formulation of gravity, in particular in the covariant phase space approach. Substantial progress has also been made studying the properties of diffeomorphism-invariant observables in classical gravity. This close connection between work on classical and quantum gravity is likely to continue in coming years, with useful ideas flowing in both directions.
